The funny thing is, that boiler has always used a 7" flue pipe and doesn't even have a clean-out tee on the back...it never has more than 1/2" or so of fly ash in it. My EFM on the other hand is hooked up with the recommended 8" pipe, dedicated baro & chimney, draft verified with gauge, etc, and it gets LOTS of fly ash in the pipe if not cleaned out 2x per season. I am not sure if it is due to a difference in boiler design (Highboy vs. DF520) or if the smaller pipe & different chimney is what makes the difference. Regardless, I think it is kind of ironic that the non-code install will run a full year with no issues, and the code install will start venting CO into the basement if the pipe isn't cleaned every 6 months or so.
I guess my point is that there is no replacement for proper maintenance.